Optical measuring apparatus



Jan. 19, 1954 E.'ROOT I1I OPTICAL MEASURING APPARATUS v2 Sheets-Sheet 1 Filed Dec. 2'7, 1948 in vemioi' Wm $1. Mm Rm A Z W, W. W WW.

Jan. 19, 1954 001- 111 2,666,267

OPTICAL MEASURING APPARATUS Filed Dec. 27, 1948 2 Sheets-Sheet 2 V I V, OUTPUT OF PH: TOGEZ J.

5 (J4EE 14/41/5 k /A//=ur 7a COUA/TEE IN V EN TOR.

K. MAL

Patented Jan. 19, 954

" UNITED STA ES "PATENT OFFICE- V or'rroAL m sm elmminsi v 1 alm rt ot'nr, Springfield, VI:. 7 Application December 27,1948,SerialNo.67,277

' 10 Claims.

.The. present inventionmelates'. measuring apparatus, and moreparticularly to apparatus for -accurately measuring-and calibrating mechanicaldevices. 1 M

In ..my oo-pending application"v Serial .No. 768,300 filed .August "13,. 1947,, now. Patent No. 2,604,004, I have described a highlyaccurate measuring .system involving vthe. principles of optical interference. In. essence the. invention described therein .comprises means for effecting an accurate. .1measurementby'-, automatically counting the. number of wavelengths. of light betweentwopoints. 1 p

The principal object .of .the presentinvention "is to provide asubstantially continuous record of the deviation from standardof 'all portionsof a mechanical piece. Forexample, alead screw, which is. tobe usedin. a;pr'ecision machine tool to be accurately calibrated so that its accuracy throughout its length may be known. .'lhe cali.- bration of a leadscre'vi willl be .used forthe spe: cific description of "this invention, although .the invention is applicable tothe calibration o'fother parts, as will be apparentjtothose skilled in the. 25

art. .With the foregoing and other. obj ects..in.- view, as will hereinafter appear, .the principal feature ofthe invention comprisesa suitable drive for thepart to be calibrated, .a record sheet drivenin correlation therewithand ,optical means to measure. the motion ofsaidpart, with. means for markinggthe record sheet in accordance with point-to-point measurements .onjsaid; part. in

- termsof wavelengthsof.light;- Inthe preferred form, of the invention the recording. means comprises a traveling shee t, in.combinationwith one ormore traverse electrodes which are e er ized at ..definitefintervals, which intervalsfare meas-- ured in terms of. a definite. count" of wavelengths. Preferably. the motions are. so correlaltedthat' a perfect. part .will vresultlin. a straight-line. record along .the sheet,. while, variations; from. standard are indicated .by sloping lines. ;.Thus in the .case of. .a-lead. screw, periodic or random variationsin pitch are represented-by aflwavy line, .whilea cumulative error is represented by a. series. of slopinglines... I

Dtherjeatures of the invention. consist of certain novelfeaturesof construction, and combina-.' tions. and. arrangements fjof partaghereinafter :described and aru many defined. in the. claims. In the accompanying drawings, Fig. I is a, dia: :gram of thepreferred fo'rm orgapparams" accord-j ing. liq thepre'snt inv'ntionl; f 2 is a diagram iilliistrating the wave forms at different parts ofthe apparatus and Figs. 3, 4 and arereprejsentations of typical records obtained by the apparatus.

The illustrated embodiment of the invention comprises apparatus for accurate calibration of a lead screwvof a type. to be used in a precision lathe. H j Asshownvin Fig. .1. a. bed 2 carries bearings .4 and Sin which are. mounted .the screw 8 whose accuracy is to be measured. A carriage II] is adapted to move along bed 2 in. adirectionparallel to'the. axis of screw. 8. To reduce friction the carriage l0 may roll on balls indicated at 12, mounted in suitable carriers or races, not shown. Aifollower I 4 may be resiliently mounted on carriage I0 bymcans of .reedsl5. The follower l4 may take the form of a full or partial nut en-i gaging the screw.. Alternatively, the screwmay be mounted ina'fixedhut, engaging carriage Ill withits end so that the entire screw advances on.

rotating and moves carriage. I 0 with it.

. Aplane refiectingsurface J6 is mountedon carriage. Ill atright angles to theaxisof screwB and preferably with that .axis passing through its center... The surface 16 is part of an.inter.-'

ferometer. The vremainingparts of the inter;- ferometerare mounted on a carriage l8 which is adapted tov move along the bed 2 in a direction parallel to the axis of the screw 8. Theinterferometer comprises, in addition to the surface.

l6, a source of sharp spectral lines. 20,. an aperture 22, a collimating lens 24, a prism 26, a plane half-reflecting surface 28, a plane reflecting surface 3l l, an objective'lens 32, an aperture 34, and a photocell .36. The source 20 may be, for example a krypton lamp provided with a supply of,

direct current. The reflecting, surface is placed so that its image in surface'28 is. accurately'parallelto the surface l6, and may be:

, emittedby source 20 to. enter aperture 34.

met fifiidi e re 8 'thr u h e'ntiail gear 40, a gear ratio selection unit 42, an u nu sha tfl d an u te cnn. ea A sprocket is driven from shaft 43 through bevel. gears .46, 41, atfarate which bears a constant ratio to the rotation of the screw. Sprocket 45 engages a strip of recording paper 48 which is fed from a roll 58. The paper 48 may carry coordinate lines 52 based on degrees of rotation of screw 8.

A disc 54 is also driven by the motor 38. Disc 54 carries one or more marking devices 56, 58, 60 which project downward toward the surface of paper 48 as it passes over an electrode plate 6|. The marking devices are adapted to make dots on the paper through impulses applied thereto; preferably they comprise pins to which electrical impulses are applied, and the paper 48 is electrical recording paper. motor shaft connects with the disc. through suit-. able gearing, here shown as bevel gearing 82, 63, 64.

The carriage I8 is normally fixed with. respect to bed 2 while the machine is operating. As screw 8 rotates, causing the carriage l8 to-"move along the bed 2, the intensity of the light reaching photocell36. varies. in a sinusoidal manner. by reason of the optical interference effects, which require no further description. The signal from the photocell 38 is fed to the grid of tube 65. Tube 65 together with tube 65. is connected in a direct coupled trigger circuit which converts the sine wave signal from the photocell to a square wave. The square wave output is taken from the plate of tube 86 and fed into a differentiating circuit comprising condenser .68 and resistance 10.. The differentiating circuit converts the square wave into a series of alter nate positive and negative pulses which are fed into an electronic counter 12 of conventional.

construction. The character of the wave is shown diagrammatically for the several points in Fig. .2. The counter 72 preferably registers pulses of onlyone polarity, for example, negative pulses, and is arranged to give a positive output pulse upon each registration of a given number, for example 5.0., of input pulses. Thus, the counter 12 may be designated a scale-of-o counter, and its actual construction may take any suitable form, as willbe understood by those skilled in the art. An important function of the trigger circuit containing tubes 65 and .65

is. to prevent registration of false pulses due to noise in the photocell circuit. This is accomplished by separating the trigger tripping potentials for increasing and decreasing signals by an amount greater than the noise amplitude.-

The output pulses from counter 12 are applied to the grid of a discharge tube 14, causing a condenser 16 to discharge through the primary of a transformer 18. Current from the secondary of transformer 18 passes to the disc 54 through a brush 80, and thence through one of the recording pins (56, 58, 60), and through the paper 48 to the grounded electrode plate 6|, leaving a mark on the paper. A mark may thus be made. on the paper every time carriage I0 ads vances a distance equal to 50 half. wavelengths of light. Thus the counter 12 may be considered to. be a wavelength counter since it is efiective to mark the paper upon motion of the carriage in multiples of wavelengths of light.

The ear ratio of unit 42 is preferably so chosen in relation to the pitch of screws 8 that if screw 8 is perfectly accurate, the recorded-marks Qn'paper 48 will lie in a straight line parallel to the direction of motion of the paper. In general such a gear ratio is difiicult to obtain by simple gearing because of. incommensurability; and will further be subject to slight modification rom time to time due 'to such. factors as tem- To drive the disc, the

perature, air pressure, and humidity. In order to make it possible to use simple gearing in unit 42 and to make possible the various corrections in ratio, a multiplier unit 82 has been provided. The multiplier 82 is a continuously acting mechanical multiplier of conventional construction, such as a variable speed drive, having a constantly driven input shaft 86 driven from the motor 38 through suitable gearing, and a rate adjustment shaft 8.! represented by a hand crank. The =multiplier is arranged so that the angular position of its output shaft 84 repre .sents the product of the angular position of input shaft 8.6..and a constant (the angular position of the input shaft attached to handwheel .511), -1The yarious correction factors are added and entered on the liandwheel 81 prior to operation of the machine. The multiplier 82 may take any well-known form, such as the ball-anddisk or the sliding-linkage type, and is not illustrated indetail.

Thev output of multiplier .82 is added to .the input of gear unit 42 through the differential gear 40. In general the contribution of multi plier 8.2 to the. input of unit. 42 will be very small in proportionv to the direct contribution of motor 38. so that the multiplier 82 is not necessarily of high accuracy.

For a given pitch of screw 8 there are a number of ratios for the gear unit 42 which will produce record lines parallel to the direc i of motion of the. paper 48 if the screw is a p rfect one. point on the. paper is not corre ed a s lu y with any particular startin point on the it is essential that the reco d line or line b permitted to s art anywhere n the sheet. Furthermorc, in case. of drift sufiici n t ar y h record line beyond the. bound e of h e the. invention provide for introduct n f other record line wh be c n id red to piece on to the first on. Thi may be s n from. Figs. 1 and 4.

As a particular example. assume h th ear ratios are such that the disc 54 makes of a turn while the carriage I8 moves a distance of 50 half-wavelengths. Assume further that the counter 12 is on a scale. or '50, and that the disc 54 rotates counterclockwise and has three equally spaced recording pins. It is also necessaryin this example that the sheet 48 span at least half the distance between two successive recording points; in other words, the sheet 48 (or the recording portion thereof) should span a chord of about 60 of the circumference of the disc. A

' dot will be made on the paper at a point indicated in Fig. 3 at a. when one of the r ordin pins (say pin 56) .is over the paper. Assume that this point is at the center of the sheet although it need not be. The nest impulse will come, when the dischasmoved 60 and there will be no recordingv pin over the paper. The next impulse will come when the disc has moved 60 more and the pin 5.8 is over the p per- If the screw is perfect in the interval between these two points, the second point will be at b 'on the same straight line. I

If on the otherhand the first recording p n 5B1makes its flrst marl; c .at the extreme upp r edge of the sheet as shown the next impulse will .cause the same 911155 to make a d t don the low r d e of the sh et- Thenext tw impulsese and f will be made by the pin 58 at the top and bottom of the sheet respectively. The points 0, e, etc. will form areccrd line at the However, since the lateral position of a top edge of the sheet, while the points at, f, etca will forma line at the bottom, if the screw'is perfect. r l 7 Ifthe screw is not perfect, the dots will drift one side or'the other as shown inLFig. 3. In the i case where the record line starts in the center of the sheet, or near it, as shown'in Fig. '3,'and

the deviations are not large, only a single line will appear. However, if the line is near the edge of the sheet, or in the extreme case where will result in the introduction of a continuing record at the other side of the sheet. Thus, in Fig. 1, the record indicates that the screw has too coarse a pitch sincesuccessive dots are made earlier and earlier in the cycle of the revolution of the disc. runs off the upper edge of the sheet and starts at the bottom edge. In the case of too fine a pitch, the record will be as indicated in Fig. 4 wherein the record continually runs off the sheet at the lower edge.

Since in this case a line is formedby alternate impulses, the longitudinal spacing between successive dots on any line corresponds to a carriage motion of 100 half-wavelengths. Of course the ratios of gears 44, 46, 4! may be so chosen as to give a paper travel either greater or less than the actual carriage travel.

The lateral spacing between points is determined as follows: In the foregoing example, the gear ratio 42 is such that the disc 54 revolves of a revolution for a carriage advance of 50 halfwavelengths. Thus, the distance between the extreme lines in Fig. corresponds to a measure. of 50 half-wavelengths. The deviations of the screw from perfection may therefore be. deter mined by scaling the deviations of the record from a straight line, a

It is, however, not necessary to use the foregoing ratio. As another example the ratio may. be such that the disc 54 revolves /6. l ln for 50 half-wavelengths advance of the carriage, .In

that case, referring to Fig. 3, they first dot would,

be made by pin 56 and the secondone by pin 60. The record wouldbe in all respects similar to that obtained for the other ratio, except that the distance between the edges of the sheet would correspond now to a distance of only ten half-.- wavelengths. In other words, by increasingthe speed of the disc, the sensitivity of the record is increased. Furthermore, it is not necessary to use three pins. One, two, or in fact any number of recording pins may be used. Itwill be seen,

that many possible combinations of gear ratio $2, with various numbers of recording pins will suffice to give a straight record line for a perfect screw. However certain arrangements which will normally be preferable for reasonsof simplicity and economy maybe found by application of the following general conditions: The recording pins will be uniformly spaced; the width W of the recording portion of the paper will be such that not more than one pin will be-inrecordijng noel-- tion at any one time; thus eliminating the need correspondingto every second recording pulse.

The record line thus continually '70 for a clrcuitcapable. of recording simultaneously at two or more points; at leastv one and not more (this makes full use of the capacltyliof the recording circuit pursuant to condition 2).. As a consequence of the. foregoing, width W will not be less than the chord of one half the arc between two adjacent pins. The gear ratio 42 will be such that disc 54 turns 112/212 of a revolution between successive recording impulses where m is any odd integer and n is the number (odd or even) of recording pins (m may be greaterv or less than 2n).

If the above'conditions are followed, it will be found that the separation between two record lines. (corresponding approximately to W will correlated if a straight' line record is'to be obtained. It is for this reason that the multiplier 82 is provided. The multiplier iscapable of producing, in effect, continuous changes in gear ratio, 1 and thus permitting a straight graph to .be made in spite of incommensurability between the dimensions of the screw and the wavelength of the light. The multiplier 82- may be'omitted, however, in which case a perfect screw may result in a record like that of Fig.4;in which the successive lines are inclined, and deviations from standard are represented by variations in slope. It will be understood that, owing to the rotary motion of the disc, the lines will not appear straight unless a large number of pins is used. However, by'taking account of the curvature, an effective calibration 'may be obtained. Such a record, although accurate, is less convenient to use than a graph in which the incommensurable factors have been eliminated.

In the event that the screw to bemeasured is longer than the continuous range over which the interferometer will give-sharp fringes, the measurement'may be made in several :steps. As theinterferometer approaches theend of its range, motor 38 is stopped and carriage i8 is moved to a new position. .The instrument is then ready for the start ofa new step. In general there will be a discontinuity of the record every time that carriage i8 is moved, however the total error ,of the screw maybe obtained by addingtogether theyerrors; obtained separately from the continuous part of. the record for each step. In some cases it may not be necessary to stop motor 38'between steps, since if carriage l8 is moved quickly enough-the error in the screw "which would have been shown by the short por tion' of the record thus lost maybe negligible.

From the foregoing description, it will be seen that the invention provides a simple and exceptionally accurate :means for making a complete calibrationrecord of: the-deviations of a mechanical piece from-standard Although the preferred form ofthe invention has been shownand de scribed, the inventionis not limited to the particular embodiment herein described but may be varied within the purview of the appended claims. furthermore,- the invention'is not lhnited to calibration of a lead. screw butmay he applied to a cam or other mechanical part, the ,motion, ofwhich is utilized to effect motion of "a part :of the interferometric measuring system.

= Having thus describedthe invention, I claim:

1. Measuring apparatus'comprising afollower to engage a piece to be measured, drive means ;for causing relativei motion' between said piece and the follower, dptic'al means for accurately measuring thefi near displacement caused by said motion in terms of wavelengths of light, a re. corder having means for moving a record sheet in accordance with the motion of the drive means, a marking device for the recorder, means for effecting a traverse of the marking device relative to the sheet at a rate determined by the motion of the drive means, and means for applying successive activating impulses to the marking device at regular displacement intervals of said relative movement between the piece and the follower, to cause successive points to appear on the record sheet, one coordinate of their position being dependent on the motion "of the drive means and the other on the relative motion of the piece and its follower.

2. Measuring apparatus comprising a follower to engage a piece to be measured, drive means for causing relative motion between said piece and the follower, optical means for accurately measuring the linear displacement-caused by said motion in terms of wavelengths of light, a recorder having means for moving a record sheet in accordance with the motion of the drive means, a rotary marking device for the recorder, means for rotating the marking device to cause it to travel generally crosswise of the sheet at a rate determined by the motion of the drive means, and means for applying successive activating impulses to the marking device at regular displacement intervals of said relative movement between the piece and the follower, to

cause successive points to appear on the record.

sheet, one coordinate of their position being dependent on the motion of the drive means and the other on the relative motion of the piece and its follower.

3. Measuring apparatus comprising a follower to engage a piece to be measured, drive means for causing relative motion between said piece and the follower, optical means for accurately measuring the linear displacement caused by said motion in terms of wavelengths of light, a recorder having means for moving a record sheet in accordance with the motion of the drive means, a marking device for the recorder having a recording pin arranged to traverse the sheet at a rate determined by the motion of the drive means,

and means for applying successive marking im-v pulses to the pin upon relative movement of said piece and follower through predetermined displacement intervals.

4. Measuring apparatus comprising a follower to engage a piece to be measured, drive means for causing relative motion between said piece and the follower, optical means for accurately measuring the linear displacement caused by said motion in terms of wavelengths of light, a recorder having means for moving a record sheet in accordance with the motion of the drive means, a marking device for the recorder having an electrical recording pin, means for causing the pin to traverse the sheet at a rate determined by the motion of the drive means, and means for applying successive electrical impulses to the pin upon relative movement of said piece and follower through predetermined displacement intervals.

5. Measuring apparatus comprising a follower to engage a piece to be measured, drive means for causing relative motion between said piece and the follower, optical means for measuring the linear displacement caused by said motion in terms of wavelengths of light, a wavelength 8' counter, arecording device having means to mark a record sheet, and connections between the counter and the marking means to deliver a marking impulse thereto at successive intervals measured by a predetermined wavelength count.

6. Measuring apparatus comprising a follower to engage a piece to be measured, drive means for relatively moving said piece and the follower, an interferometer of which one element moves with the follower to form a succession of interference fringes during said relative motion, counting means for said fringes, means for generating an impulse upon each count of a predetermined number of fringes, a recording device having means for moving a record sheet and a marking device, means for causing the marking device to traverse the sheet, and connections to deliver said impulses to the marking device.

7. Measuring apparatus comprising a follower to engage a piece to be measured, drive means for relatively moving said piece and the follower, an interferometer of which one element moves with the follower to form a succession of interference fringes during said relative motion, counting means for said fringes, means for generating an impulse upon each count of a predetermined number of fringes, a recording device having means for moving a record sheet, a marking device including a rotary member having a marking pin, and connections to deliver said impulses to the pin.

8. Measuring apparatus comprising a follower to engage a piece to be measured, drive means for relatively moving said piece and the follower, an interferometer of which one element moves with the follower to form a succession of interference fringes during said relative motion, counting means for said fringes, means for generating an activating impulse upon each count of a predetermined number of fringes, a recording device having means for moving a record sheet, a marking device including a rotary memher having a number of marking pins spaced in uniform angular relation thereon, and means to deliver said activating impulses to said pins.

9. Measuring apparatus comprising a follower to engage a piece to be measured, drive means for causing relative motion between said. piece and the follower, a recorder having means for moving a record sheet in accordance with the motion of the drive means, a marking device for the recorder, gearing associated with the drive means to cause the marking device to traverse the sheet, optical means for measuring the relative motion between the piece and follower, means for applying successive activating impulses to the marking device upon measurement by the optical means of a linear displacement of a predetermined number of wavelengths of light, to cause successive points to appear on the record sheet, one coordinate of their position being dependent on the motion of the drive means and the other on the relative motion of the piece and its follower, and means for varying the effective gear ratio between the drive means and marking device to fix a desired ratio between the displacement caused by the relative motion and the displacement of the marking device.

10. Calibrating apparatus for a screw comprising drive means for the screw, a follower to be linearly driven by the screw, an interferometer having an element 'moved by the follower to cause light variations by optical interference, counting means for said light variations, impulse generating means operable upon each count of a predetermined number of light variations, a

9 recording device having means for moving a record sheet, connections from the screw drive means to the recording device to move the sheet at a predetermined rate, a marking device mov able laterally of the direction of motion of the sheet, and connections from the impulse generating means to the marking device to apply marking impulses to the sheet, whereby successive record points appear on the sheet having coordinates dependent on the drive means and on the relative motion between the screw and follower. V

ELIHU ROOT III.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date ,208,725 Balderston Dec. 19, 1916 Number 10 Number Name Date Garms et a1 Apr. 1 8, 1933 Sawford May 12, 1936 Keinath May 21, 1946 Young Feb. 4, 1947 Marrison Dec. 30, 194'? Wilson et a1 Jan. 13, 1948 Snyder Feb. 22, 1949 FOREIGN PATENTS Country Date Germany Mar. 7, 1933 

